1. Technical Field
This invention relates to chemical analysis, and more particularly to alkylation process analysis and control.
2. Background Information
Introduction to the Refining Alkylation Market
Between a quarter and a third of the world's refineries operate alkylation units, which convert relatively low-value byproducts of the crude oil refining process into alkylate, a high octane component used to make gasoline. Among the numerous control variables that determine the economics of alkylation is the composition of the acid catalyst. Globally, the number of alkylation units using hydrofluoric acid (HF) is currently about 125 versus about 90 using sulfuric acid (H2SO4 or SA). Slightly more than half of all alkylation units in the world are located in North America, where gasoline is favored over diesel as a motor fuel for passenger cars and alkylate is accordingly a valued blending component.
Another important application of alkylation technology is in the production of LABs (linear alkyl benzenes), important as a raw material used in laundry detergents. However, the total number of alkylation units in operation to produce LABs, as well as tonnage produced, is rather small compared with the refining industry.
Background: Alkylation Process Control
Alkylate is one of the most important gasoline blending components in the refining industry. Because it has an extremely high octane number, contains virtually no sulfur, and can be produced using olefinic by-products from the fluidized catalytic cracking (FCC) unit, alkylate has been called refiners' gold. Given that the reactants are seldom pure, and that propylene is sometimes mixed with the olefin feed, alkylate in practice comprises a mixture of compounds instead of pure isooctane as depicted in the following idealized equation:Isobutane+Isobutene→3,3,5-trimethypentane(Isooctane)
Produced in a continuous-flow process, the chemical addition of isobutane and isobutene is effected conventionally through liquid phase catalysis involving strong acids such as hydrofluoric acid (HF) and sulfuric Acid (H2SO4, or SA), although solid phase catalysts are currently under development.
Monitoring and controlling the composition of the liquid acid catalyst, i.e., acid strength and the levels of impurities that dilute the acid, are among the most important challenges associated with the profitable operation of the alkylation process. One important impurity is water, which enters the process with the feed streams. Though present at ppm (parts per million) levels, water accumulates in acid catalyst at percent levels due to feed rates ranging from a few thousand barrels per day (bpd) to tens of thousands of bpd. By contrast, acid soluble oil (ASO, as defined hereinbelow) accumulates in the acid catalyst, a by-product of reactions involving feed impurities that contain sulfur, oxygen, or conjugated double bonds.
HF Alkylation
In the case of HF alkylation (HFA), water generally is controlled at levels below 2% to minimize corrosion of equipment in the unit. Also, total hydrocarbons dissolved in the catalyst are typically held at levels around 11%-16% to yield alkylate of the required quality and maximize process economics. In HFA, HF strength is controlled through acid regeneration within the alkylation unit, which is essentially a distillation process that separates HF from the higher-boiling impurities, H2O and ASO (Acid Soluble Oil, as defined hereinbelow).
If HF strength drops below about 80%, side reactions can accelerate and lead to a condition called acid runaway, which consumes HF and produces large amounts of ASO. Such runaways rarely occur, as unit operators usually have time to detect the incipient runaway and “pull charge” (withhold olefin feed) to stop the process before the runaway condition actually occurs. However, this action also stops the production of alkylate while the catalyst is regenerated. Furthermore, acid regeneration itself has associated costs including energy required to run the unit, neutralization and disposal of hydrocarbon byproducts, and the addition of fresh, pure HF. Thus, the ability to monitor and control catalyst composition in real time allows refiners to avert runaways, reducing operating costs while also tending to maximize product quality (octane), throughput, and the time between maintenance shutdowns to repair or replace corroded components.
SA Alkylation
SA alkylation (SAA) differs from HFA in that the catalyst generally is not, with rare exception, regenerated on site at the refinery. HF has a relatively low boiling point and can be distilled. By contrast, SA is essentially non-volatile and therefore cannot be purified through distillation. Rather the “spent” acid generally must be shipped by rail car for remote processing. Thus, the high cost associated with off-site regeneration partially offsets the perceived safety advantage of SA over HF, i.e., its low volatility.
Given that the alkylation reaction occurs only when acid strength is sufficiently high to catalyze the reaction of isobutane with olefins, the effectiveness of SA diminishes when its strength falls below a certain level due to accumulation of ASO and H2O— typically around 88%-90%. Thus, the economics of SA alkylation depend on knowing exactly the point where SA becomes too weak and must be taken out of service. For example, taking SA out of service when its strength is 89% may be very costly if good quality alkylate can be produced economically with acid strength≧88.5%.
Traditional Analysis of Acid Catalyst
The composition of acid catalyst is typically determined by manually obtaining a sample for analysis in the local refinery laboratory daily, weekly, or several times each week. In contrast with hydrocarbon samples routinely analyzed in the refinery lab, full analysis of acid catalyst samples tends not to be straightforward due to special requirements for sample handling, preparation, and analysis. Additionally, HF presents a safety hazard due to its volatility and toxicity. With both HF and SA, comprehensive determination of composition is difficult for at least two reasons. First, measurement of water generally depends on a Karl Fischer titration method specially modified to neutralize the strong acid. Second, ASO is not a single compound, but includes a range of chemically-related compounds that have a rather wide range of molecular weights and boiling points, some of which (e.g., “light ASO”) can evaporate rapidly at room temperature.
Analysis Frequency
In consideration of the foregoing difficulties, refiners may test the acid as infrequently as possible to minimize laboratory workload. Some refiners make do with one analysis per week while refineries operating in Los Angeles County, Calif. may be required by regulation to test HF catalyst once every 8 hours. Infrequent analysis may be sufficient to permit process control under stable operating conditions, but not to identify rapid changes caused by occasional surges in feed impurities that lead to generation of ASO.
Analytical Reproducibility and Completeness
Compounding the issue of analysis frequency, laboratory test results may not always be reliable due to the difficulty of obtaining a representative sample when sample volumes are minimized in consideration of safety, as may be done in the case of HF catalyst. This further compounds the difficulty of reproducibly executing the test method itself. And if technicians running the tests do not routinely perform the Karl Fischer water measurement, acid strength measured by titration may be the only parameter known in regard to catalyst composition, severely limiting operators' ability to optimize the process.
Safety
As mentioned, HF is both volatile and toxic. Sampling, sample handling, and testing therefore are executed in accordance with audited procedures carefully designed to ensure the safety of operators and technicians. In the case of HF, testing frequency may be deliberately suppressed to minimize exposure risks.
All of this underscores the undesirability of manual methods for routine analysis. Attempts have been made to replace manual sampling and testing with online measurement techniques, to facilitate the efficient and safe operation of alkylation units. To date, however, these attempts have generally been unsatisfactory, e.g., due to incomplete or inaccurate measurements by simple univariate instruments; or due to excessive complexity, lower-than-desired reliability and/or relatively high costs, such as associated with conventional use of spectrometric technologies. Thus, a need exists for an improved analyzer system for real-time alkylation process analysis and control.